Why is iron the threshold beyond which heavy-nucleus FISSION releases energy, but nuclei lighter than iron release energy via FUSION? (Use the binding energy per nucleon curve to answer.)

1. Energy is released in a nuclear reaction whenever the PRODUCTS have a higher TOTAL binding energy than the REACTANTS. 2. The BE/nucleon curve peaks at $A \approx 56$ ($\,^{56}\mathrm{Fe}$) at $\sim 8.8\,\text{MeV}/\text{nucleon}$. 3. LIGHT nuclei (e.g. H, D, He) are LEFT of the peak — they have low BE/nucleon. FUSING them into a heavier nucleus moves products CLOSER to iron, INCREASING average BE/nucleon → energy released. 4. HEAVY nuclei (e.g. uranium, plutonium) are RIGHT of the peak. FISSIONING them into two mid-mass fragments ALSO moves products closer to iron → BE/nucleon increases → energy released. 5. Therefore iron is the "valley of stability": every spontaneous reaction tends to push nuclei toward $\,^{56}\mathrm{Fe}$, the most tightly bound region. 6. Option A states the OPPOSITE (iron has the HIGHEST, not lowest, BE/nucleon). Options C and D are unrelated misconceptions. _Source: NCERT Class 12 Physics Part 2, Ch 13, §13.7 (Nuclear Energy — fission and fusion both arise from the BE/A curve), p. 9–11._